The present invention relates generally to a refractive-index logging device and more particularly to a downhole refractive-index logging device for evaluating gas or oil-bearing strata.
Since most of the known gas and oil remaining in the earth is in tight reservoirs (i.e., the product cannot be easily removed without enhanced recovery techniques), these tight reservoirs have become a major target for gas and oil supplies to meet present and future needs.
Exploration, development, and production of natural gas and oil are known to be more effective and less costly when quantitative evaluations of earth formations are accurate. However, the production parameters (porosity, saturation, permeability, etc.) of tight reservoirs are difficult to resolve. Therefore, accurate logging tools with high-resolution capability are needed for their cost-effective production.
For the past three decades induction and dielectric logging tools have been used to evaluate some properties of earth formation around a borehole. The immediate function of these logs is to determine the electrical parameters (conductivity and dielectric constant) of formations when downhole probe responses are obtained in terms of volts and amperes. Their principle of operation is that gas and oil bearing formations have smaller electric conductivity and dielectric constant than formations saturated by underground water. However, measurements with these logs in tight reservoirs result in various ambiguity problems such as contradictory, erroneous, or unresolvable measurements because the composition and structure of formation materials in tight reservoirs is much more complex than in conventional wells. Accordingly, logging tools capable of interrogating more comprehensive parameters are needed to provide more accurate resolutions.
One system for radio frequency dielectric induction well logging is shown in U.S. Pat. No. 4,012,689 of Percy Cox et al. This patent discloses a 30 MHz transmitting antenna and a plurality of receiving antennas spaced along a downhole logging device, each antenna comprising a single coil of electrostatically shielded wire. The purpose of the shielding is to suppress undesirable electromagnetic modes induced at the feed terminal of each antenna. However, the shielding introduces additional coupling modes which also affect the usefulness of the resulting measurements. Cox calculates his results from the theory of a point source oscillating magnetic dipole, assuming a constant magnetic permeability .mu..
U.S. Pat. No. 4,278,941 of Robert Freeman solves some of the problems caused by borehole mud and invasion effects through the use of four simple (i.e., conventional loop) receiving coils. However, the simple coils of this patent must be tuned to each frequency and do not permit multiple-frequency, swept CW or broadband pulse operation permitted by a conical double-helix antenna. In addition, the patent fails to provide for logging-mode purity through the use of a unique connection (i.e., balanced center-feed terminal) between the coaxial cable and the antenna. Finally, the patent does not provide for the evaluation of magnetic permeability, requiring users of the patented technique to assume a value and hope that the actual value of permeability equaled the assumed value.
U.S. Pat. No. 3,449,657 of Oke A. Fredriksson et al. discloses a helical antenna for irradiating a salt formation in the earth with electromagnetic energy in a plane transverse to the axis of a borehole to explore for oil reservoirs around a salt dome. The antenna consists of a cylindrical conductive tube having conductors spaced from the tube by insulating posts. The conductors emanate from a central location along the tube with two codirectional conductors spiraling towards each end position. (Codirectional windings are each wound in the same direction such that they do not cross each other.) The conductors are connected to a coaxial cable by a duplexer coupler, enabling the antenna to be used for transmitting and receiving. The 15 foot long antenna is used as a logging tool in arid salt formations, where it excites a travelling-wave mode with a narrow beam to determine the distance to electromagnetic discontinuities. This antenna would be useless in the wet environment of a tight reservoir which requires a magnetic-dipole mode in a broad beam to determine the refractive indices of formation surrounding a borehole.